As mentioned the '3 for 1' offer is the best you'll get atm, think it's $10 and then whatever your bank/paypal turn that into in £'s,
Also, the cost, you can see on their website what the basic content is, it's not too bad but it's only '3' road cars (one has a rookie and advanced version, another has a 'cup' and 'roadster' version, so kinda 5 but not) and 2 oval cars (again, one has a rookie/advanced versions so 3-ish), and there's a few tracks.
Now to start with that's not too bad, the rookie series which are the only ones you'll be able to race immediately after signing up (you've got a licence, safe driving = higher licence = more series available) only use the basic content so they're free to race in, any higher series will cost varying amounts depending on what content is needed, and what offers you get (10% off for 3 pieces at a time, 20% off for 6 pieces at a time, and not really applicable here but 25% off if that purchase takes you to 100% of the content owned).
The basic cost for stuff is, iirc, $11.95 for a car and either $11.95 or $14.95 for a track. A years subscription is $99 with upto $40 back in 'participation credits' over that time.
So it's expensive really, I've got 100% content, and using iRacing prompted me to get triple monitors, and a G25, and 1 major/1 minor PC upgrade since 2008, and I'm now looking at another major upgrade with my main game still being iRacing, seriously expensive stuff. But worth it...
The software itself is, imo, the best simulation software out there, not perfect by any means but simply the best available (to us commoners at least), and with the season structure so you get new builds every 3 months it does keep getting better, slowly but surely.
But the main appeal is the overall structure, as a driver you've got 2 main ratings (well 4, 2 for road, 2 for oval, I'm ignoring 'ttRating' here as it's a pointless mess), these are:
iRating (how good you are, gain some for finishing high up in a race, lose some for finishing at the bottom, if a race has > 1 'split's worth of drivers then iRating determines your split.
Safety Rating: This is the main one, determines your licence level by promotions/demotions, and tries to ensure that races are mostly clean, rookie races specifically are kinda messy but it does get better trust me, and it's infinitely better than the public stuff in LFS/rFactor
Um, think that's about it, any questions ask or post in the thread linked
